Similar labeling patterns of threonine, isoleucine and methionine were obtained for metabolism of 14C-aspartate and 14C-homoserine in Lemna, arguing against channeling of aspartate into separate threonine and methionine branches. Labeling patterns obtained with these two isotopes in plants supplemented with threonine demonstrated little or no feedback regulation of threonine synthesis, in contrast to the marked feedback regulation by each of the other members of the aspartate family (methionine, isoleucine, lysine) on its own synthesis. This finding corroborates previous studies in which endogenous rate of threonine synthesis were determined by the extent of dilution of specific activity of 14C-threonine fed exogenously, and agrees with our failure to establish any regulatory property of threonine synthase that would allow specific control of the threonine biosynthetic branch. Systematic studies on the growth effects of the 7 possible combinations of threonine, methionine and lysine, in conjuntion with labeling patterns from 14C-aspartate and 14C-homoserine, revealed only one condition of "crossregulation", i.e. inhibition of methionine biosynthesis by supplementation with threonine plus lysine. Studies have recently been initiated on aspartokinase, which catalyzes the first step in the entry of aspartate into the aspartate family of amino acids. We developed a sensitive and specific assay for aspartokinase, which is currently being employed in examining the activities and properties of this enzyme from plants growing with a variety of amino acid supplements. Our research begins to reveal an unusual regulatory role of threonine whereby this amino acid regulates its own synthesis only under conditions of lysine overproduction and, in so doing, can deprive the plant of methionine. These findings, together with more recent tentative results, now allow us to propose a novel and logical strategy for the use of genetic engeneering in improving the nutritional quality of methionine and lysine in plants.